Upgrade fails with no usable explanation

[Danny Sauer]

Attempting to run "do-release-upgrade -p" because the signature of a plain "do-release-upgrade" has a purportedly invalid signature. This is an upgrade from an up-to-date 12.05.5-LTS to 14.04.1-LTS, pulling packages through squid-deb-proxy on another machine, using the command-line updater over ssh. The updater starts up, switches software sources, pulls down the new information, and then exits, telling me only:

...
Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.
...

This is not particularly helpful, as it doesn't give me any clue whatsoever as to what packages are potentially conflicting, or if the problem is even on my end. It would be nice if the tools indicated that there were debug logs somewhere, or game me a hint as to what packages are involved, or drew an ASCII-art guy on a bicycle from a 1990's usenet signature block, or /something/ beyond simply taking its ball and going home.

I suspect that there's an issue with using the postgres packages from the postgres repository rather than the official Ubuntu packages. But that's just a wild guess based on previous conflicts rather than anything based on what's really going on. I'll eventually track down what the problem is here, but meanwhile this is, I guess, a feature request for somewhat more usable failure messages.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.17
ProcVersionSignature: Ubuntu 3.2.0-65.99-generic-pae 3.2.60
Uname: Linux 3.2.0-65-generic-pae i686
ApportVersion: 2.0.1-0ubuntu17.8
Architecture: i386
Date: Tue Jan 13 18:26:15 2015
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2015-01-14 (0 days ago)